Anal. Chem. 2000, 72, 642-643
Response to Comments on Adsorption versus Absorption of Polychlorinated Biphenyls onto Solid-Phase Microextraction Coatings SIR: In earlier work1 we reported that adsorption becomes more important than absorption for controlling SPME collection of PCBs from water as their molecular weight increases (e.g., above ∼250 amu). Vaes et al. contend that we misinterpreted our data and explain our data by proposing that our experiments suffered from substantial (as much as ∼90%) losses of the PCB solutes to a Teflon coated stir bar during our 5-hour SPME sorption step. To determine if such losses occurred, we replicated our earlier PCB sorption studies using the identical experimental conditions with stir bars, and with mixing achieved by external agitation provided by a vibrating engraving tool. Within experimental error, no substantial losses of the PCBs to the stir bar were found, thus demonstrating the arguments of Vaes et al. do not apply to our previous work. We appreciate the concerns which Vaes et al. express over our report that SPME/water partitioning of higher molecular weight PCBs is controlled more by adsorption rather than absorption processes.1 Their assertions that PCB losses to the stir bar need to be controlled are certainly valid, and careful considerations of these potential losses need to be applied in any SPME study. Vaes et al. make several complex arguments on the basis of our data1,2 to support their ideas, but (unfortunately) do not provide any experimental evidence. In the discussion below, we report new results determining the loss of PCBs to stir bars during SPME sorption and discuss the major points raised in their paper. PCB Losses to the Stir Bar during SPME. In essence, the arguments of Vaes et al. require that substantial losses of PCBs must occur to the stir bar used to mix the water during SPME equilibration. According to their Table 1 calculations, the losses of each congener range from 20 to 92% for low to high molecular weight PCBs, respectively. We have conducted additional experiments to directly determine if such losses to the stir bars used in our earlier study do occur. SPME equilibrations were performed using a new 100-µm PDMS fiber and the same PCB solution (50 ppt of each congener in water), with all other conditions identical to those † Present address: Department of Chemistry, East Carolina University, 205 Flanagan, Greenville, NC 27858-4353. (1) Yang, Y.; Hawthorne, S. B.; Miller, D. J.; Liu, Y.; Lee, M. L. Anal. Chem. 1998, 70, 1866. (2) Yang, Y.; Miller, D. J.; Hawthorne, S. B. J. Chromatogr., A 1998, 800, 257.
642 Analytical Chemistry, Vol. 72, No. 3, February 1, 2000
in our original study.1 One set of sorption experiments was performed with new stir bars exactly as before.1 An additional set of sorption experiments was performed with no stir bar in the vial, the vial being mixed by securing it to an engraving tool and violently agitating it during SPME sorption. All equilibrations with the stir bar and with the engraving tool were performed for 5 hours,1 and seven replicates were performed for each method (one determination for each method per day). For the arguments of Vaes et al. to be correct, the quantities of the higher molecular weight PCBs sorbed by the SPME fiber must be as low as ∼10% (corresponding to a 90% loss in their Table 1) with the stir bar present compared with the quantities sorbed by the same fiber when the mixing was achieved by external agitation. However, the results shown in Table 1 demonstrate that there is little, if any, difference in the amount of each PCB congener found, regardless of whether mixing was performed by a stir bar or by external agitation. Although the standard deviations of the raw areas are somewhat high (likely because the determinations were spread out over 7 days due to the 5-hour equilibration used for each method), certainly the 20-90% losses to the stir bar required by the arguments of Vaes et al. do not occur. Therefore, their arguments are disproved, and our earlier conclusions that adsorption increasingly controls SPME partitioning of high molecular weight hydrophobic solutes (like PCBs) appear to be correct. Other Comments. In addition to concerns of PCB losses during SPME partitioning, they point out that the best way to determine Kd values would be to determine the mass of each PCB congener in the SPME fiber and in the water after SPME sorption. We certainly agree, when such determinations are possible. Unfortunately, for the wide range of PCB congeners used in our study, determination of PCB concentrations in the water phase after SPME sorption is simply not practical. First, very low concentrations of PCBs must be used (50 pg/mL in our study) to avoid exceeding the water solubility for many congeners. Second, while all of the molecules in the SPME fiber can be introduced into the GC column, only a tiny fraction of PCBs left in the water phase (e.g., after extraction with hexane) can be injected for analysis. Thus, since ∼10 to 80% of the PCBs in the water are removed by the SPME needle, determination of the remaining few picograms per milliliter in the water after SPME sorption is not realistic. For example, if the water was extracted with 100 µL of hexane followed by a 1-µL on-column injection, the detector would have to be capable of reliably quantitating
